Sensor element for detecting at least one property of a measuring gas in a measuring gas space, containing a ground, impregnated slip layer

ABSTRACT

A method for manufacturing a sensor element for detecting (i) a gas component in a measuring gas or (ii) a temperature of the measuring gas includes: introducing at least one functional element into at least one slip at least once in such a way that a slip layer is applied to the functional element, the functional element including at least one solid electrolyte and at least one functional layer; sintering the slip layer on the functional element; grinding the slip layer at least in the area of the at least one functional layer; impregnating the slip layer; and thermally treating the impregnated slip layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sensor element and a method fordetecting a measuring gas.

2. Description of the Related Art

A large number of sensor elements and methods for detecting at least oneproperty of a measuring gas in a measuring gas space are known from therelated art. In principle, this may involve arbitrary physical and/orchemical properties of the measuring gas, whereby one or multipleproperties may be detected. The present invention is described below, inparticular, with reference to a qualitative and/or quantitativedetection of a gas component of the measuring gas, in particular, withreference to a detection of an oxygen content in the measuring gas. Theoxygen content may, for example, be detected in the form of a partialpressure and/or in the form of a percentage. Alternatively or inaddition, other properties of the measuring gas are also detectable,however.

For example, such sensor elements may be designed as so-called lambdasensors, as are known for example, from Konrad Reif (publisher):Sensoren im Kraftfahrzeug [Sensors in the motor vehicle], 1^(st)edition, 2010, pp 160-165. With broadband lambda sensors, in particularwith planar broadband lambda sensors, it is possible, for example, todetermine the oxygen concentration in the exhaust gas in a large area,and thereby deduce the air-fuel ratio in the combustion chamber. The airratio λ describes this air-fuel ratio.

Ceramic sensor elements, in particular, are known from the related art,which are based on the use of electrolytic properties of certain solidbodies, i.e., on ion-conductive properties of these solid bodies. Thesesolid bodies may be, in particular, ceramic solid electrolytes such as,for example, zirconium dioxide (ZrO₂), in particular, yttrium-stabilizedzirconium dioxide (YSZ) and/or scandium-doped zirconium dioxide (ScSZ),which may contain small additional amounts of aluminum oxide (Al₂O₃)and/or silicon oxide (SiO₂).

Such sensors are subject to increasing functional demands. Inparticular, a rapid operational readiness of lambda sensors after anengine start plays a significant role. This readiness is influencedessentially by two aspects. The first aspect relates to a rapid heatingof the lambda sensor to its operating temperature above 600° C., whichmay be achieved by a corresponding design of a heating element or by areduction of the area to be heated. The second aspect relates to therobustness against thermal shock as a result of water hammer during anoperation. The aforementioned thermal shock is due to the fact that fora certain period of time after the engine start, the temperature in theexhaust pipe lies below the dew point for water, so that water vaporformed during fuel combustion is able to condense in the exhaust pipe.This leads to the formation of water droplets in the exhaust pipe. Dueto the impact of water droplets, the heated ceramic of the lambda sensormay be damaged or even destroyed as a result of thermal stresses orfractures in the sensor ceramic. For this reason, lambda sensors havebeen developed which have a porous ceramic protective layer on theirsurface, also referred to as a thermal shock protection layer. Thisprotective layer ensures that water droplets impacting the lambda sensorare distributed over a wide area, thereby reducing the locally occurringtemperature gradients in the solid body electrolyte or sensor ceramic.Thus, in the heated state, these lambda sensors tolerate a certaindroplet size of condensed water without being damaged. The protectivelayer is normally applied to the sensor element in an additional methodstep. Various materials such as, for example, aluminum oxide or spinel(MgAl₂O₄) and coating techniques such as, for example, spray processesor immersion processes are used for this purpose.

In spite of the numerous advantages of the methods for manufacturingsensor elements for lambda sensors known from the related art, there isnevertheless potential for improvement.

BRIEF SUMMARY OF THE INVENTION

Therefore, a method for manufacturing a sensor element for detecting atleast one property of a measuring gas in a measuring gas space, and asensor element manufacturable according to this method are provided,which at least largely avoid the disadvantages of known methods andsensor elements, and in which the robustness against thermal shock maybe improved using a cost-effective method.

The method according to the present invention includes the followingsteps, preferably in the sequence cited, whereby in principle anothersequence is also conceivable, however:

-   -   introducing, in particular, immersing at least one functional        element at least once into at least one slip in such a way that        a slip layer is applied to the functional element, the        functional element including at least one solid electrolyte and        at least one functional layer,    -   sintering the slip layer on the functional element,    -   grinding the slip layer at least in the area of the at least one        functional layer,    -   impregnating the slip layer, and    -   thermally treating the impregnated slip layer.

Moreover, the method may include one or multiple additional steps whichare not mentioned. In addition, individual or multiple or all methodsteps may be carried out simultaneously, chronologically overlapping orrepeatedly.

The functional element may be introduced, for example, by immersion intothe slip. Immersion into the slip may occur, in particular, completelyor also only partially. The functional element may be introducedrepeatedly into the slip. At least one drying process may be carried outbetween the repeated introductions of the functional element into theslip. Impregnation may be carried out with the aid of aprecious-metal-containing and/or getter-containing solution. Forexample, the impregnation may contain platinum, palladium, rhodiumand/or include a getter-containing preparation such as, for example,LiOH, MgCl₂. Prior to introduction into the slip, a cavity forming layermay be applied to the functional element. After sintering and grinding,the at least one slip layer may have a thickness of 50 μm to 600 μm,preferably of 150 μm to 350 μm, and even more preferably 200 μm to 300μm, for example, 250 μm. The slip layer may be sintered together with afunctional layer present in the unsintered state. It is equallyconceivable, however, for the slip layer to be applied to a previouslysintered functional element and to be subsequently burned in. The slipmay be, in particular, a highly fluid immersion slip capable of formingdrops, i.e., a slip based on organic solvents or which is water-based.In particular, the slip may be capable of forming drops and be filledwith oxidic solids such as, for example, aluminum oxide, zirconium oxideand/or titanium oxide, pore forming agents such as, for example,vitreous carbon or wax, fine-particle precious metal powders orfine-particle salts such as, for example, metallic platinum powder,palladium powder, rhodium powder or, for example, chlorides or nitratesthereof, fractions of binders and organic additives such as, forexample, wetting agents, dispersants, defoamers for adjusting therheological properties, solvents or water. Such slips are described, forexample in published German patent application document DE 28 52 647 A1and European patent document EP 0 386 027 B1, and their formulas,compositions and methods of preparation are incorporated by referenceherein. For example, the slip may be composed as follows: 40.0% byweight of butylcarbitol as a solvent, 1.5% by weight of polyvinylbutyralas a binder, 2.0% by weight of polyethylene (PE) wax as a pore formingagent, 0.5% by weight of a wetting agent, 42.0% by weight ofyttrium-stabilized zirconium dioxide (YSZ), and 14.0% by weight ofaluminum oxide.

The immersion coating with the slip may take place, for example, bysimple or repeated immersion with intermittent drying, different slipformulas being advantageously used during repeated coating, for example.The slip layers may, for example, include a porosity increasing from aninner to an outer layer. Each application of a slip layer may befollowed by a drying process, such as for a period of less than one hourat temperatures of less than 250° C. Following the application of allslip layers, a subsequent sintering may be carried out at a temperatureof 1200° C. to 1450° C.

Grinding may take place, for example, using a corundum grinding belt ora grinding disk. This offers the advantage of grinding multiple times.For example, grinding may take place above an outer electrode ormeasuring electrode of a lambda sensor or over a gas entry hole of abroadband lambda sensor. The areas of the slip layer having a greaterthickness may, for example, delimit or define an underlying cavity.Grinding may take place at least in the area of the at least onefunctional layer, i.e., in an area which overlaps the functional layerin a direction of a layer structure of the sensor element.

Impregnation may take place, for example, using a platinum-containingand/or rhodium-containing impregnation solution. For example, a dripprocess may be used for applying the precious metal-containing solutionto the grinding site, in which a targeted, local wetting occurs onlyabove the electrode due to a savings of precious metal. Impregnation mayalso take place, for example, using a getter-containing solution. Animmersion method is also conceivable, however, in which the ground sliplayer is immersed into the impregnating fluid. In this case, the surfaceproduced by grinding has a higher absorption capacity for theimpregnating fluid than the adjacent unground areas. The effect of thisis a lower porosity and absorption capacity for the impregnating fluidon the unground surface and a high absorption capacity for theimpregnating fluid caused, for example, by a higher open porosity at theground site. Finally, the impregnated slip layer undergoes a thermaltreatment such as, for example, a single baking of the impregnation, anda functional test on the sensor element.

As a particular variant of the method according to the presentinvention, a cavity forming layer may be applied prior to a slipcoating, for example, on the electrode side above a gas entry hole,provided, for example, with the aid of screen printing. The cavityforming layer may, for example, be a highly filled vitreous carbon pastewhich leaves behind a cavity after sintering. Thereafter, the slip maythen be dip coated, followed by a grinding process.

A sensor element according to the present invention includes afunctional element, which includes at least one solid electrolyte and atleast one functional layer, and at least one impregnated slip layer onthe functional element, the slip layer being ground at least in the areaof the at least one functional layer. The at least one slip layer mayhave a thickness of 50 μm to 600 μm, preferably from 150 to 350 μm, andeven more preferably of 200 μm to 300 μm, for example 250 μm. The sliplayer may have an open porosity of 10% to 60%, preferably of 15% to 50%and even more preferably of 15% to 30%. The slip layer may have aporosity gradient, the porosity gradient increasing from the side of theslip layer facing the functional element in the direction of the side ofthe slip layer facing away from the functional element. A cavity may besituated between the slip layers in the functional element. Thefunctional element may include a layer structure having at least onefirst electrode, having at least one second electrode and having thesolid electrolyte, the solid electrolyte connecting the first electrodeand the second electrode, the second electrode being formed separatelyfrom the measuring gas space by at least one layer of the layerstructure, the second electrode being connected to the measuring gasspace via at least one gas entry path, the gas entry path including atleast one gas entry hole in the layer structure, the cavity beingsituated between the gas entry hole and the slip layer.

The layer structure may, for example, be formed in such a way that thefirst electrode and the second electrode are situated on opposite sidesof the solid electrolyte, for example, on opposite sides of a solidelectrolyte layer such as, for example, a solid electrolyte foil or asolid electrolyte paste. Alternatively or in addition, however, the atleast two electrodes may also be situated on the same sides of the solidelectrolyte. The electrodes and the solid electrolyte form togetherpreferably at least one cell. The sensor element may be designed as asingle cell sensor element with just one individual cell, which may beused, for example, as a Nernst cell or also as a pump cell.Alternatively, the sensor element may also be designed as amulti-cellular sensor element having several of such cells, which mayalso implement different functions. For example, at least one pump celland at least one Nernst cell may be provided.

At least one of the at least two electrodes, hereinafter also referredto as the second electrode is situated in the interior of the layerstructure without weighting or sequencing these electrodes. In otherwords, the second electrode is formed separately from the measuring gasspace by at least one layer of the layer structure. In particular, thisat least one layer may be at least one solid electrolyte layer. The atleast one second electrode is thus situated in a deeper layer level ofthe layer structure, i.e., in a layer level which is formed remotelyfrom a surface of the solid electrolyte facing the measuring gas space.The at least one additional electrode, i.e., the at least one firstelectrode according to the nomenclature used herein, may also besituated in a deeper layer level; it may, however, also be situatedabove, i.e., for example, on a surface of the layer structure facing themeasuring gas space. For example, the first electrode may be designed asan outer electrode and may be separated from the measuring gas space,for example, solely by a gas-permeable porous protective layer, andotherwise, for example, is in direct gas exchange with the measuring gasspace. Various embodiments are possible.

The at least one second electrode in this case is connected to themeasuring gas space via at least one gas entry path. A gas entry path isunderstood in general to mean an element via which an exchange may takeplace between the measuring gas space and the second electrode, wherebya complete gas exchange or also merely an exchange of individual gascomponents may be ensured. For example, the gas entry path may includeone or multiple holes, channels, openings or the like. In particular,the gas entry path may be designed in such a way that it ensures asubsequent flow and/or subsequent diffusion of gas to the secondelectrode from the measuring gas space or in the opposite direction, forexample, a subsequent flow and/or a subsequent diffusion of oxygen. Thegas entry path includes at least one gas entry hole in the layerstructure.

A gas entry hole in this case is understood to mean an opening whichextends through the layer structure, in particular, the solidelectrolyte along an axis, in particular, through the at least one layerwhich separates the at least one second electrode from the measuring gasspace. The gas entry hole may, in principle, have an arbitrary crosssection, for example, a round cross section or a polygonal crosssection. The gas entry hole may, in particular, run perpendicularly tothe layer levels of the layer structure, and may, for example, have acylindrical shape, at least in sections, for example, a plaincylindrical shape.

The at least one second electrode may be situated, in particular, in anelectrode cavity. This electrode cavity may be situated in an interiorof the layer structure and may be formed, for example, as an opencavity. Alternatively, this electrode cavity may also be completely orpartially filled with a gas-permeable, porous material, for example,with a gas-permeable aluminum dioxide. The electrode cavity may, inparticular, be connected to the gas entry hole via at least onediffusion barrier. In this case, therefore, the gas entry path to the atleast one second electrode includes the gas entry hole, the diffusionbarrier or a channel in which the diffusion barrier is situated, as wellas the electrode cavity.

A diffusion barrier is understood within the scope of the presentinvention in general to mean an element which prevents, or at leastslows a direct subsequent flow of gas out of the gas entry hole into theelectrode cavity. Thus, a diffusion barrier is an element which providesa high flow resistance, whereas a diffusion of gas or gas componentsthrough the diffusion barrier is comparatively easy. The diffusionbarrier may, for example, include a porous ceramic element, inparticular, a fine-pored aluminum oxide. If such a diffusion barrier isprovided, it is in particular preferable if the diffusion barrier isdesigned recessed in relation to the gas entry hole. A recesseddiffusion barrier in this case is understood to mean a diffusion barrierwhich is not directly adjacent to the gas entry hole, but rather isrecessed in relation to the hole. For example, the diffusion barrier maybe situated in a channel or in some other opening which is a part of thegas entry path, whereby, however, the diffusion barrier does not reachthe transition between this channel or this opening and the gas entryhole directly, but rather ends at a distance from this transition. Theadvantage of this recessed or retracted diffusion barrier is that duringmanufacture of the gas entry hole, this barrier is not damaged, as aresult of which contamination of the diffusion barrier could occur or asa result of which irregularities could occur when setting the limitcurrent, which is determined by the width of the diffusion barrier. Inaddition, the aforementioned design improves stability during continuousoperation, in particular with respect to a sooting, for example, byparticles made of ash such as, for example, oil ash and/or metal oxides.

In the manufacturing method for a sensor element, the layer structuremay be manufactured by using sheeting techniques and/or thick filmtechniques and/or other ceramic layering techniques.

As mentioned previously, an introduction, in particular, an immersion,may occur completely or only partially. The functional element may be aceramic solid electrolyte present in a sintered state. However, it isequally conceivable for the electrolyte to be present in the unsinteredstate or in the annealed or pre-sintered state. Sintering may occur insuch a way that the functional element is present in the unsinteredstate and is sintered together with the applied slip layer.

A slip is understood within the scope of the present invention to mean afluid, pulpy to viscous water mineral or solvent-mineral mixture, whichmay also be referred to as a compound, for manufacturing ceramicproducts.

A functional element is understood within the scope of the presentinvention to mean an element which includes at least one solidelectrolyte and at least one functional layer. A solid electrolyte isunderstood within the scope of the present invention to mean a componentwhich is based on the use of the electrolytic properties of certainsolids, that is, on the ion-conductive properties of these solids. Thesesolids may include, in particular, ceramic solid electrolytes such as,for example, zirconium dioxide (ZrO₂), in particular yttrium-stabilizedzirconium dioxide (YSZ) and/or scandium-doped zirconium dioxide (ScSZ),which may contain small additional amounts of aluminum oxide (Al₂O₃)and/or silicon oxide (SiO₂). A functional layer is understood within thescope of the present invention to mean an element which is selected fromthe group composed of: electrode, conductor path, diffusion barrier,diffusion gap, reference gas channel, heating element, Nernst cell andoxygen pump cell. In particular, it is understood to mean those elementswhich fulfill the essential chemical and/or physical and/or electricaland/or electrochemical functions of a lambda sensor. The functionalelement may be present in the unsintered or pre-sintered state.Correspondingly, the functional element may be a finished functionalelement or a preliminary stage thereof, which must still be sintered.

An impregnating fluid or impregnating solution is understood within thescope of the present invention to mean a fluid or solution whichfacilitates the adjustment of the normal position in the slip layer, aswell as the function as a protective layer against erosive or corrosiveeffects from the exhaust gas. The impregnating fluid may be based onprecious metals. The precious metals, in particular from the platinumgroup, catalyze the adjustment of the thermodynamic balance and therebyset the sensor element normal position near the stoichiometric point,i.e., at λ=1. Also conceivable, however, are non-precious metal-basedgetter-containing solutions, i.e., materials as getters for harmfulsubstances such as, for example, lead, silicon, phosphorus, zinc, whichcould adversely affect the electrode function, and which take effectfrom the exhaust gas. Also possible is the use of mixed oxides based onat least one alkaline oxide or alkaline earth metal oxide on the onehand and a thermally stable oxide of an element having the valence of atleast three, preferably based on groups IIIa, IIIb or IVb of theperiodic table of elements. Thus, non-precious metal-based getters suchas, for example, LiOH, MgCl₂, are also possible as impregnating fluid.

A pore forming agent is understood within the scope of the presentinvention to mean any material which is constituted to render a layer orcomponent containing this material porous or lighter. For example, thepore forming agent may be contained in a slip in order to give it acertain porosity. Examples of pore forming agents are vitreous carbon,sawdust and cork dust, starch, carbon dust, polymer beads or polymerfibers, in particular, short fibers. In particular, it is understood tomean carbon-based materials which combust during so-called sintering andleave behind cavities in the process.

Porosity is understood within the scope of the present invention to meanthe ratio of the cavity volume to the total volume of a substance ormixture of substances as a dimensionless measured variable. Thismeasured variable may be indicated, in particular, in percentages. Inthis case, the open porosity is understood to mean the portion of thecavity volume of those cavities as part of the total volume which is incontact with one another and with the ambient air.

A cavity forming layer is understood within the scope of the presentinvention to mean a layer made of at least one material, which may bepreferably cleanly removed by chemical processes such as, for example,hydrolysis, solvent extraction, and/or thermal processes such as, forexample, burn-off, debinding, sintering. This material may, for example,contain a pore forming agent, which combusts during sintering. These arestarch, carbon dust, or polymer beads, for example. In particular, thisis understood to mean carbon-based materials, which combust duringso-called sintering and leave behind cavities in the process. Carbondust in the form of lamp black, for example, may be used as a cavityforming agent for manufacturing planar lambda sensors. Purely organiccomponents and/or a carbon modification may also be used such as, forexample, graphite, vitreous carbon and carbon black.

The method according to the present invention for manufacturing a sensorelement is easily adaptable to various functional element lengths. Inparticular, a thick and dense protective layer of a planar sensorelement may be obtained on all sides, i.e., a so-called all-roundprotection, in particular, of all edges in the hot area of the sensorelement. Moreover, a precise porosity setting is possible by adjustingthe slip composition, the slip preparation conditions, the layerthickness and/or the sintering conditions. A single coating or repeatedcoating is also possible. On the whole the method according to thepresent invention is cost-effective.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a view of a cross section perpendicular to a direction of alayer structure of a sensor element according to the present inventionto which a slip layer is applied.

FIG. 2 shows a view of a cross section perpendicular to a direction of alayer structure of a sensor element according to the present inventionto which three slip layers are applied.

FIG. 3 shows a view of a cross section parallel to the direction of thelayer structure and parallel to a longitudinal extension direction ofthe sensor element according to the present invention having a markingfor a grinding site.

FIG. 4 shows a view of a cross section parallel to the direction of thelayer structure and parallel to a longitudinal extension direction of amodified sensor element according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a view of a cross section perpendicular to a direction of alayer structure of a sensor element 10 according to the presentinvention. Sensor element 10 depicted in FIG. 1 may be used to verifyphysical and/or chemical properties of a measuring gas, whereby one ormultiple properties may be detected. The present invention is describedbelow, in particular, with reference to a qualitative and/orquantitative detection of a gas component of the measuring gas, inparticular with reference to a detection of an oxygen content in themeasuring gas. The oxygen content may be detected, for example, in theform of a partial pressure and/or in the form of a percentage. However,other types of gas components are, in principle, also detectable, forexample, nitrogen oxides, hydrocarbons and/or hydrogen. Alternatively orin addition, other properties of the measuring gas are also detectable,however. The present invention may be used, in particular, in the fieldof automotive engineering, so that measuring gas space 12 may, inparticular, be an exhaust system of an internal combustion engine, andthe measuring gas may, in particular, be an exhaust gas.

Sensor element 10, as an exemplary component of a planar lambda sensor,includes a functional element 14 having a solid electrolyte 16 in theform of a ceramic solid electrolyte layer 16 and having a functionallayer 18. Functional layer 18, for example, is an outer electrode ormeasuring electrode of a lambda sensor. In general, functional element14 may have a layer structure, in which, for example, solid electrolyte16 is constructed of multiple electrolyte films. One or multiplefunctional layers 18 may be situated, for example, between and on theseelectrolyte films such as, for example, a heating element and multipleelectrodes.

Sensor element 10 also includes an impregnated slip layer 20. Slip layer20 may be situated, for example, in the form of a drop on solidelectrolyte 16. Slip layer 20 may cover the entire surface or a portionof the surface of solid electrolyte 16. Slip layer 20 is ground, atleast in the area of the at least one functional layer 18. Slip layer 20may, for example, have a thickness of 50 μm to 600 μm, preferably of 150μm to 350 μm, and even more preferably of 200 μm to 300 μm, for example,250 μm. Slip layer 20 contains, in particular, oxidic solids, inparticular, aluminum oxide, zirconium oxide and/or titanium oxide. Sliplayer 20 also contains finely dispersed precious metals such as, forexample, platinum, palladium, rhodium. Slip layer 20 may have an openporosity of 10% to 60%, preferably of 15% to 50%, and even morepreferably of 15% to 30%, for example, 20%. For example, slip layer 20may have a porosity gradient. The porosity in this case may increasefrom a side 22 of slip layer 20 facing functional element 14 in thedirection of a side 24 of slip layer 20 facing away from functionalelement 14.

In particular, slip layer 20 is impregnated. The impregnation may beintroduced, for example, by a precious metal-containing and/orgetter-containing preparation during manufacture of sensor element 10,as is described in greater detail below. Slip layer 20 acts as a thermalshock protection layer, the impregnation ensuring that functionalelement 14 is not choked by harmful substances of the measuring gas,because the harmful substances from the exhaust gas such as, forexample, silicon, adhere to or adsorb on the impregnation and thereforedo not reach functional layer 18. Moreover, the precious metals act as acatalyst in order to decompose non-combusted components of the measuringgas. The aforementioned porosity ensures that per time unit only aspecific amount of measuring gas passes out of measuring gas space 12 tofunctional layer 18.

Sensor element 10 may be manufactured, in particular, as describedbelow.

A functional element 14, which includes at least one solid electrolyte16 and at least one functional layer 18, is initially introduced into aslip. For example, functional element 14 may be immersed just once intothe slip. In this way, a slip layer 20 is applied to functional element14. Functional element 14 in this case may be introduced completely orpartially into the slip. As shown in FIG. 2, functional element 14 mayalso be introduced repeatedly into the slip. In this case, three sliplayers 20 are applied to functional layer 14, as shown in FIG. 2. Threeslip layers 20 shown in FIG. 2 may in this case be made from the sameslip or from different slips. For example, the slips may differ in termsof the amount of pore forming agent and layer thickness. Thus, the slipsmay be used, for example, in order to adjust the porosity gradient in aslip layer 20 formed from multiple slip layers. Thus, for example, theporosity may increase from a side 20 of slip layer 22 facing functionalelement 14 to a side 24 of slip layer 20 facing away from functionalelement 14.

The slip may, for example, be a highly fluid immersion slip capable offorming drops, in particular based on an organic solvent or water-based.The slip may, in particular, be filled with oxidic solids such as, forexample, aluminum oxide, zirconium oxide, titanium oxide, pore formingagents such as, for example, vitreous carbon or wax, fine particleprecious metal powder or precious metal salt such as, for example,platinum powder, palladium powder, rhodium powder or, for example,chlorides or nitrates thereof, fractions of binders and organicadditives such as, for example, wetting agents, dispersants, defoamingagents for adjusting the rheological properties, solvents or water.

Functional element 14 may include at least one ceramic solid electrolyte16 and at least one functional layer 18. For example, functional element14 is present in the unsintered state or as already sintered functionalelement 14. For this reason, unsintered solid electrolyte 16 and sliplayer 20 applied thereto may be sintered together. If functional element14 is immersed repeatedly, an intermittent drying may take place betweenthe individual immersing operations. In such case, drying may takeplace, for example, for a period of less than one hour at temperaturesbelow 250° C. Sintering may take place at temperatures between 1200° C.and 1450° C.

Subsequently, slip layer 20 is then ground, at least in the area of theat least one functional layer 18. Grinding may take place with the aidof a corundum grinding belt or a grinding disk. This offers theadvantage that sensor elements 10 may also be ground multiple times.

FIG. 3 shows a marking 26 at which grinding may take place. Inparticular, marking 26 indicates a grinding plane. Once ground, sliplayer 20 exhibits a resulting layer thickness of 50 μm to 600 μm andpreferably of 200 μm to 300 μm, for example, 250 μm. For example, sliplayer 20 may be ground on one side above an outer electrode asfunctional layer 18 of a lambda sensor, or above a gas entry hole of aplanar broadband lambda sensor. Above in this case indicates a layerlevel, which is situated above functional layer 18 in a direction asseen from functional element 14 to measuring gas space 12 perpendicularto the layer structure of sensor element 10.

This is followed by an impregnation process with, for example, aprecious metal-containing preparation and/or a getter-containingsolution. For example, an impregnating fluid may be applied to sliplayer 20 at least in the area of the ground site with the aid of a dripprocess. For example, the impregnating fluid is applied in the form of atargeted, local wetting only above functional layer 18 due to a savingsof precious metal, for example, with a platinum-containing andrhodium-containing impregnating fluid. Alternatively, however, animmersion method may be used in which functional element 14 and groundslip layer 20 are immersed into the impregnating fluid. The surface ofslip layer 20 produced by grinding has a higher absorption capacity forthe impregnating fluid than the adjacent non-ground areas. Accordingly,more impregnating fluid penetrates the ground areas of slip layer 20than the non-ground areas.

This is followed by a thermal treatment of impregnated slip layer 20such as, for example, a single baking, in order to fix the impregnationin slip layer 20. The method is concluded by carrying out a functiontest on sensor element 10.

FIG. 4 shows a view of a cross section parallel to the direction of thelayer structure and parallel to a longitudinal extension direction of amodified sensor element 10 according to the present invention.Hereinafter, only the differences relative to the aforementioned sensorelement 10 are described. Sensor element 10 of FIG. 4 may be part of aplanar broadband lambda sensor and includes a cavity 28 above afunctional layer 18, which is delimited by slip layer 20. Functionallayer 18 may, for example, be a gas entry hole. Cavity 28 may beproduced by applying a cavity forming layer to an unsintered functionalelement 14 or an already sintered functional element 14, for example,using a cavity paste applied with the aid of a screen printing process.The cavity forming layer may, for example, include a highly filledvitreous glass paste. This is followed by applying slip layer 20 in thesame manner as described above. During sintering, the cavity forminglayer combusts, preferably residue-free, and leaves behind cavity 28.Slip layer 20 is ground at marking 26. The course of marking 26 in thiscase shows that by grinding slip layer 20, cavity 28 is exposed on oneside facing measuring gas space 12, so that the measuring gas has freeaccess to the gas entry hole. It is possible, however, for grinding tobe carried out in such a way that cavity 28 remains separated frommeasuring gas space 12 by a thin slip layer 20, so that the measuringgas is able to pass through the pores in slip layer 20 to the gas entryhole. Following this are the above-described method steps of applyingthe impregnation, the thermal treatment of impregnated slip layer 20 andthe function test of sensor element 10.

The manufacture according to the present invention of sensor element 10is clearly apparent by viewing sensor element 10 and with supportingmaterial analysis of sintered slip layer 20.

1-14. (canceled)
 15. A method for manufacturing a sensor element fordetecting a gas component in a measuring gas or a temperature of themeasuring gas, comprising: introducing at least one functional elementat least once into at least one slip in such a way that a slip layer isapplied to the functional element, the functional element including atleast one solid electrolyte and at least one functional layer; sinteringthe slip layer on the functional element; grinding the slip layer atleast in the area of the at least one functional layer; impregnating theslip layer; and thermally treating the impregnated slip layer.
 16. Themethod as recited in claim 15, wherein the functional element isintroduced repeatedly into the slip.
 17. The method as recited in claim16, further comprising: at least one drying process between the repeatedintroductions of the functional element into the slip.
 18. The method asrecited in claim 15, wherein impregnation is carried out with the aid ofat least one of a precious metal-containing solution andgetter-containing solution.
 19. The method as recited in claim 15,wherein a cavity forming layer is applied to the functional elementprior to introduction into the slip.
 20. The method as recited in claim19, wherein the at least one slip layer, after sintering and grinding,has a thickness between 150 μm and 350 μm.
 21. The method as recited inclaim 15, wherein the functional element is introduced into the slip inan unsintered state, and the functional element and the slip layer aresintered together.
 22. The method as recited in claim 19, wherein thefunctional element is introduced into the slip in a sintered state, andthe slip layer is sintered on the functional element.
 23. A sensorelement for detecting a gas component in a measuring gas or atemperature of the measuring gas, comprising: a functional element whichincludes at least one solid electrolyte and at least one functionallayer; and at least one impregnated slip layer on the functionalelement, wherein the slip layer is ground at least in the area of the atleast one functional layer.
 24. The sensor element as recited in claim23, wherein the at least one slip layer has a thickness between 150 μmand 350 μm.
 25. The sensor element as recited in claim 23, wherein theslip layer has an open porosity between 15% and 50%.
 26. The sensorelement as recited in claim 25, wherein the slip layer has a porositygradient increasing from a side of the slip layer facing the functionalelement in the direction of a side of the slip layer facing away fromthe functional element.
 27. The sensor element as recited in claim 26,wherein a cavity is provided between the slip layer and the functionalelement.
 28. The sensor element as recited in claim 27, wherein thefunctional element includes a layer structure having (i) at least onefirst electrode, (ii) at least one second electrode, and (iii) the solidelectrolyte, wherein the solid electrode connects the first electrodeand the second electrode, the second electrode being formed separatelyfrom the measuring gas space by at least one layer of the layerstructure, and the second electrode being connected to the measuring gasspace via at least one gas entry path which includes at least one gasentry hole in the layer structure, and wherein the cavity is situatedbetween the gas entry hole and the slip layer.